Flat solar thermal panels or evacuated tube?

06-01-2009, 11:43 PM

First off, great forum..been reading the past hour and now I have a question. We want to put solar panels on our roof to heat our water. Is it better to go with the flat panels or the evacuated tubes? We want the most of it because it is so expensive to get it set up! Thanks in advance.

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I don't think there is really a right answer because there are different factors.

1. tubes are more efficient, but cost a lot more. The lower efficiency of panels isn't necessarily a problem, since the sun is effectively unlimited, so you can put up more panels (unless you have limited space). So you need to know the specific costs of each compared to the amount of water they heat; this varies with suppliers, but in many cases, tubes will be cheaper.

2. how robust they are. Tubes can be a little fragile, where as you can usually jump on a panel without damaging it. So are you likely to get anyone throwing rocks?

3. how accessible they are; tubes can get extremely hot. You don't want children or animals touching them.

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actually i was just reading an article that states flat panels are more efficient IF the temperature of the fluid at the panel inlet is low compared to ambient. the higher the inlet temperature, the less efficient the heat transfer is for the flat panel. the evacuated tube looses its efficiency more slowly. eventually, the to curves cross. but from my reading, it appears they end up being quite similar in performance at the temperatures people use them. the moral of the story i would guess is: get as much heat out of the system before returning it to the panels.

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It is important to understand all losses including
> Pump energy (fluid) or Fan energy (air)
> radiation losses
> distribution losses

People often confuse efficacy with efficiency. Efficacy is defined as the
capacity or power to produce a desired effect. So the proper semantics would
be which type of panel is the most effective at harvesting energy, for your unique situation.

Measure efficacy in $EH/$SC
So the best system is the one which achieves the highest $value in Energy Harvested per $System-Cost

Some folks don't care about $$$ cost, because perhaps they are either
rich or else naively idealistic : "We must generate energy via alternate sources regardless of the financial cost."
However, $$$ is how we keep score in a manner that cannot be argued with.

The best system is simply the one that is the most cost-effective. The term that is most often used is : "What is the payback period?"
If it (alternate energy) becomes a no-brainer “do-it” for the poor, then there may be hope for our planet.

We owe it to the planet to develop simple technologies for harvesting energy
from the sun/wind/uHydro, that are affordable by even the poor in our society. For this reason, it is a disservice to the planet to pay any credence to systems that are not cost effective. Energy must cease to be a commodity.

Solar Air Heating systems are more effective in sunny cold climates, whereas in a sunny warm climate, hot-water systems are more effective. In a non-sunny climate, then perhaps wind/uHydro is most effective. If you live in outer-space, or Germany, then PV. Each unique installation requires a separate comparative cost analysis.

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I experimented with Solar Air heating for about 4 years. The panel was successful in generating very hot air. The panel would heat-up by 9am, fan turn on (@ 100 degrees) & progressively get hotter. It was not uncommon to heat-up beyond the thermometer's limit of 160 degrees, with the fan blowing
(300cfm) constantly.

I had remote wireless thermometers installed on the input (cold air) & output vents. It would effectively heat-up the air from 68 to 150 degrees on sunny days.

My problem was my house has 10" front deck, with the panel that far away from the nearest window. I would lose 20-30 degrees between the panel & the room. Still it would heat the house warm enough to keep the furnace from running.

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Flat solar hot water panels have been around for close to 100 years, and evacuated tubes for more than 30 years. The flat panels deliver more BTU's per $ than evacuated tubes in clear, moderate conditions, but evacuated tubes perform better in cloudy and/or cold conditions. Doug

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2 things to think about
re snow on tubes, consider your latitude. the farther north, the higher the pitch of the array. i live at 46 degrees but have my array at 70 degrees to maximize the low winter sun. i can assure you no snow stays on the tubes at that angle. they are not as efficient in the summer, but i have alot more daylight for them to work in. i made my rack with the angle adjustable but i have not seen the need to change the angle.

if it is cold alot where you are, tubes may be better. mine were happily flowing 125 degree glycol when it was 20 below last week. they also work in cloudy weather, though not as good as when the sun is shining, of course.

these tubes will run alot hotter if you need that. i have had my array running at 170 but at the moment they are at 135.

other posts were correct. these are tubes inside of tubes, thus the vacuum. they are air temperature to the touch.

i would never put these at ground level only on a roof where errant rocks such as from a lawnmower could find them.

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Interesting. As mentioned before my attempt at solar water heating failed (copper in a flat panel). I'm wondering...you mention flowing glycol @120. Are you using the heated anti-freeze for heat or just hot water? Curious about the GPM of heated water (AV).

I'm wanting to install Pex radiant heat, but using solar to heat it. The array would need to sustain a constant 110-130 temp while pump is running.

I'm wanting constant pumping of hot water (AV) for the 6 hours of good sunlight. WHen the fluid heats to 120, pump comes on, stays on as long as sun shines.

Do you think this type (evacuates tubes) can produce that much hot water?

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Interesting. As mentioned before my attempt at solar water heating failed (copper in a flat panel). I'm wondering...you mention flowing glycol @120. Are you using the heated anti-freeze for heat or just hot water? Curious about the GPM of heated water (AV).

I'm wanting to install Pex radiant heat, but using solar to heat it. The array would need to sustain a constant 110-130 temp while pump is running.

I'm wanting constant pumping of hot water (AV) for the 6 hours of good sunlight. WHen the fluid heats to 120, pump comes on, stays on as long as sun shines.

Do you think this type (evacuates tubes) can produce that much hot water?

I'll be looking into it.

Thanks Jeff

Hello Jeff

Here is another way to look at it.
The ultimate power you can harvest from any type of panel, before losses, is approx 950W/m2, or about 88W per sq ft of collector area. Useful winter sun ~6 Hrs so ultimate daily energy harvested = 6 Hrs x 88W = 528 WH per sq ft

In Canada, typical winter heat requirement for an average dwelling averages out around 2.5 W/sq-ft, so I'm thinking that in Illinois, a reasonably well insulated house could maybe get by with 1.5W/sq ft. So a 1000 sq ft house would require 1500W x 24Hrs = 36,000 WH per day, for heat.

This is your minimum panel area, for any solar energy harvesting system [U]before losses[/U], and with every day a blue-bird day.
Of course any system has less than 100% efficiency, so you must factor that in. My direct-air panel has a measured efficiency of ~74%, so, using that number, 68.2 / .74 = 92 sq-ft of collector per 1000 ft floor-space. Round that up to 100 & you should be good.

The argument over flat-panel vs evacuated-tube vs direct-air is 100% irrelevant. The sun simply does not care... 950W/sq-m is all we get... period.
The best system is simply the one with the lowest overall losses & the quickest payback. The efficiency differences between solar-thermal panel types is minimal, and more directly related to common-sense, understanding the losses (& mitigating) & care in construction.

Vacuum tube elements (expensive) have low radiation losses in themselves, but a well insulated collector box has the same effect, perhaps for lower overall cost. Removing the harvested energy to house-inside quickly is key in minimizing radiation loss.

Vacuum tube collectors have their real place at the focus of parabolic troughs. Google "parabolic trough".

Heating the air in the space below a floor (if there is such a space) has the same net effect as directly heating the floor itself. My basement has pipes & things that must not freeze, so the basement needs heat anyways. Our main-floor surface temperature is exactly the same as the basement air below. I chose a direct-air collector, because my mission was to solar-contribute to heating my basement air, & thus the house.

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re evacuated tubes and radiant floor and my prior post. my 75 tube array shares a boiler loop with a backup boiler. on this loop is a radiant loop for 450 sq ft, a heat exchanger for a 2700 gallon endless pool (indoors) and heat exchanger for dhw. the tubes run during the day. the boiler runs after the sun sets to catch up if there is a need. I only let the pool take boiler heat for an hour max via a wiring setup and relay i have with the pool circ pump.

the boiler turns off at 8am and cools to prevent thermal shock from the tubes that start circulating about 10 am this time of year.

the solar gain(south facing, mostly glass) in this room along with the floor heat keep this room at 66. i have turned off the boiler on a really cold night (-20) to see what happened and the room temp fell to 59 so the floor holds alot of heat .(The floor has a 2" "mudpack" base under ceramic tile.)

i am running glycol as this is a closed system, not a drainback.

once the loop is warmed, even now when the sun is low the panel loop will continuously run about 120-125, and heats the pool and the floor as long as they are calling for heat.

although i have run the array at 170, it does not run that hot continuously except in the long days of the summer. I find that i can harvest the most heat running it with a differential setpoint controller set at 135/115.

i am still working on the dhw as i have a standalone hw heater for now. it has been problematic getting the aquastat to call correctly as there is no port for it on the conventional tanks. this may have to wait for the how water heater to be replaced.

finally, i have my array at 70 degrees to maximize the low winter sun. the array runs less efficiently in the summer. I rarely dump heat even in the summer with this setup.